1. Field of the Invention
The present invention relates to the field of the biological purification of wastewater such as, notably, municipal effluents, industrial wastewater as well as supply water to be made fit for drinking. It concerns more especially a method of purification in which the water to be treated and the oxygenated gas are sent in ascending co-currents in a single reactor or biological filter provided with, as a filtering material, plastic materials or expanded materials with a density lower than that of water.
2. Description of the Prior Art
It is known that the biological treatment, for example of water, consists in degrading the organic impurities by the action of a purifying biomass that is free or fixed and that contains a variety of micro-organisms such as bacteria, yeasts, protozoa, metazoa etc. In the method using free biomass, by activated sludge, the concentration, in large numbers, of the various species of micro-organisms which have little settling capacity, is impossible to achieve inasmuch as the concentration of the biomass is obtained by settling. The method is therefore limited as regards the load applicable in terms of BOD (biological oxygen demand) and COD (chemical oxygen demand). In a system with a fixed biomass, the concentration of the biomass (with the bacteria) is achieved by making the bacteria cling to a carrier medium. The settling capacity is then no longer the essential criterion and this technique possesses a purification potential far greater than that of standard methods.
Among the most efficient methods based on the principle of purification with fixed biomass, we may cite notably those patented and developed by the Applicant, among them the so-called "Biocarbone" (registered mark) method and the technique consisting in the implementation, in a single upflow reactor of a granular bed constituted by two zones having different granulometry and different biological characteristics (French patents No. 76 21246 published under No. 2 358 362; No. 78 30282 published under No. 2 439 749; No. 86 13675 published under No. 2 604 990).
In the so-called free biomass techniques, reference will be made here especially to methods using fluidized beds wherein the material used as the biofilter consists of products with a density of less than 1 such as, for example, expanded polymers, according to processes which are now in the public domain (French patent No. 1 363 510 dated 1963; U.K. patent No. 1 034 076 dated 1962), various variant embodiments of which have led to numerous invention patents (French patents Nos. 330 652, 2 406 664, 2 538 800; U.S. Pat. No. 4 256 573; Japanese patent No. 58-153 590 etc.).
The use of these floating materials and of fluidized granular beds is promising in itself but entails a number of difficulties and frequently shows drawbacks. For example, if materials heavier than water (such as sand or similar materials) are fluidized, then a considerable input of energy is needed for the pumping of the liquid and it is difficult to control the keeping of the liquid inside the reactor. To overcome this drawback of energy consumption, it has been proposed to use a fluidized bed with light materials, having lower density than water, with an insufflation of air at the base of the bed but with a supply of descending water (U.S. Pat. No. 4 256 573 and Japanese patent No. 58 153590 referred to here above). However, from a certain downflow speed of the water, the air bubbles are trapped within the material or else carried along by the liquid flow and it is not possible to aerate the reactor properly.
With a view to obviating the above-mentioned drawbacks, numerous experiments have been conducted by the Applicant to use all the advantages of a floating bed in seeking to eliminate the phenomena of the trapping of bubbles on the surface, the clogging of the bed, energy consumption, the difficulties of washing the filtering bed, etc.